During magnetic resonance image acquisition, subject motion can lead to artifacts, intensity loss, and lowered sharpness that degrade the clarity with which lesions and anatomic details can be depicted. For example, cardiac imaging sequences often span multiple heartbeats to obtain the necessary temporal and spatial resolutions. Under such conditions, motion artifacts due to respiratory motion can significantly impair the quality of the MR images (MRIs).
A number of techniques have been proposed and developed over the years to minimize the effects of motion, particularly respiratory motion, on MR images. Breath-hold techniques and navigator methods are among these approaches. Each of these techniques has its own inherent advantages and limitations. For example, breath-holding, the simplest and most commonly used approach to reducing motion artifacts, limits the available scan time and may induce physiological changes during the course of a breath-hold.
Navigator methods, on the other hand, allow normal or quiet breathing and require less patient coaching. They greatly extend the available scan time and help maintain a normal physiologic state. However, it is often difficult to accurately measure the position of organs such as the heart in real-time.
A typical MRI navigator approach to compensating for respiratory motion during cardiac imaging utilizes an excitation profile that crosses the diaphragm and generates a signal that may be used to track the superior-inferior motion (head-to-toe motion) of the diaphragm. The position of the diaphragm at its interface with the lungs is detected by the navigator signal and used to determine the heart location. Since respiratory motion of the heart is dominated by a superior-inferior (SI) component that is approximately linearly correlated to the SI motion of the diaphragm the position of the heart may be estimated. However, this approach necessitates a priori knowledge of the correlation coefficient between heart and diaphragm motion, a variable parameter among patients and different portions of the heart. For example, in one study the mean correlating factors amongst patients for the right coronary artery (RCA) root and the left anterior descending (LAD) artery vis-à-vis the SI position of the diaphragm were determined to be 0.57±0.26 (standard deviation (SD)) and 0.70±0.18 (SD), respectively.
Recent research shows that there is also an anterior-posterior (AP) movement of the heart that is correlated with movement of the diaphragm, but again in patient-specific ways. Application of navigators to the free wall of the left ventricle (LV wall) is more reliable for detecting AP motion. However, LV wall navigators have the disadvantage that magnetization voids can occur at the lung-LV free wall interface, and using a smaller diameter navigator to compensate reduces the signal-to-noise ratio (SNR) of the navigator. Application of pencil-beam navigators directly through the heart generally leads to destructive interference between the pencil beam navigators and the imaging volume, affecting image quality and navigator accuracy.